A mass spectrometer has a uniform magnetic field and an electric field. Further, the spectrometer is equipped with three spaced electrostatic quadrupole lenses. The deflection angle .phi..sub.m of the beam in the magnetic field, the radius r.sub.m of the circle described by the beam in the magnetic field, the deflection angle .phi..sub.e of the beam in the electric field, and the radius r.sub.e of the circle described by the beam in the electric field are so set as to satisfy the following relations: The spectrometer can measure masses over a wide range even if it is built as a small-sized instrument. Thus, the instrument is able to measure molecular ions of large masses.

Between the electrostatic sector (SE 23) and the magnetic sector (SM 30) of a mass spectrometer, there is provided a quadrupole (QP 26) which applies parallel beams to the magnetic sector whose inclination depends on the energy dispersion of the particles. A slotted lens (LF 27) corrects the divergence of the quadrupole in the perpendicular plane. A suitable relationship between the angle of the inlet face of the magnetic sector (SM 30) and the deflection angle provided thereby ensures that the second order aperture aberrations of the magnetic sector are corrected. The chromatic aberrations may be corrected by means of a hexapole (HP 25) centered on the focus of the quadrupole (QP 26). Another hexapole (HP 22) placed upstream from the electrostatic sector (SE 23) level with a constriction in vertical section of the particle beam serves to correct second order aperture aberrations related to the electrostatic sector (SE 23).

The disclosed mass spectrometer has, positioned between an input slit and an output slit, crossed by particles emitted by a sample, an optical coupling system placed between two respectively electrostatic and magnetic sectors. The optical coupling system comprises at least two lenses with slits oriented respectively along a first direction in which the path of the ions is incurvated by the electrostatic and magnetic sectors and along a direction perpendicular to the plane of the path. The position of the two lenses on the optical axis of the spectrometer is determined to obtain a compensation for the chromatic dispersions throughout the axis downline from the spectrometer, a stigmatic image of the input slit in the output plane of the spectrometer and a stigmatic image downline from the spectrometer.

A lens arrangement (30) for the focusing of a beam of electrically charged particles (24) in the beam path of imaging systems, more particularly in mass spectrometers (10), is indicated, the lens arrangement (30) being connected to an electrical voltage supply. The lens arrangement (30) is situated at the location or in the vicinity of the intermediate image (29) produced by the imaging system, and consists of a plurality of plates (32 to 35) disposed in succession, with aligned transmission apertures (38 to 41), the plates being connected to adjustable electrical voltages.

In various embodiments, provided are ion optics systems comprising two or more pairs of ion condensers arranged where the first member and second member of each pair are disposed on opposite sides of a first plane such that the first member of the pair has a position that is substantially mirror-symmetric about the first plane relative to the position of the second member of the pair and wherein the deflection angle of each of the ion condensers is less than or equal to about .pi. radians.